Glow discharge device



May 16, 1950 w. A. DEPP 2,507,696

GLOW DISCHARGE DEVICE Filed March :27, 194a F/G./ y f 645 FILLED 2i: I II M I l 0.6 gi 2 0.5 2 0.4 5% 957 No 574 u 0.3 SE 0.2 $2 0.1

o'l'o'z'o a o 4 o's o'6 o PRESSURE mm. or H R FIG. 5 v, a z Lw :00 I 68012 00 1650 2o oo I 24100 I zl soo I msousnlc r crass PER SECOND M Fi 3FIG 6 GAS FILLED INVENTOR v WADEPP AT TO/PNEY Patented May 16, 1 950UNITED STATES --orrics GLDVV DISCHARGE DEVICE Applicaticn March 27,1948, Serial N0.*17,52Z

1s Glaims. 1

This invention relates to glow discharge devices and more particularlyto such devices especially suitable for use in speech and other signaltransmission circuits.

In general, glow discharge devices comprise a cathode and an anodeimmersed in an ionizable medium, that is a gas or mixture of gases, at apressure such that a discharge between the electrodes can be sustainedupon maintenance of a prescribed potential difference between theelectrodes. In such devices of presently known construction, when adirect current potential sufficient to sustain a discharge is impressedbetween the electrodes, the current through the device includes spuriousvariations or noise components. Some of these components may besuppressed or prevented by adjustment or designof the external circuitassociated with the device. However, other components cannot beeliminated in this manner and, thus, impair the efficacy of the devicefor transmission of signals.

Among components of the latter type is an oscillatory noise ofsubstantial amplitude which may be of complex wave form but is none'theless repetitive. Its frequency spectrum varies from medium to medium.For example, in d vices employing argon the oscillatory noisefrequencies are in the audio range, specifically a few hundred to a fewthousand cycles. For lighter gases, these frequencie are higher. It isevident that such noise deleteriously aiiectsthe signal to noise ratiofora glow discharge device utilized in a signal transmission path.

One object of this invention is to substantially eliminate noise, andparticularly noise of the type above indicated, in glow dischargedevices.

Another object of this invention is to improve the transmissioncharacteristics of glow discharge devices and more specifically torealize a low impedance for such devices at audio fre quencies.

' It has been discovered that oscillatory noise is associated with thecharacterof the region of the discharge path immediately adjacent theanode and that this, in turn, for a given gas, is dependent upon the gaspressure and the cathode to anode spacing. Furthermore, it has beendiscovered that a definite critical relationship be tween gas pressureand cathode to anode spac ing exists for which oscillatory noise willnot be produced. Specifically, for any given gas and gas pressure, ifthe cathode to anode spacing is below a certain magnitude, oscillatorynoise will not be produced; conversely, for a given gas and electrodespacing, if the gas pressure is below a -mercury or less.

.2 certain value oscillatory noise will not obtain. The product of gaspressure and maximum cathode to-anode spacing is approximately aconstant over a range of pressures.

In accordance with one feature of this invention, accordinglyJn a glowdischarge device the gas pressure and cathode to anode spacing arecorrelated so that noise, particularly noise of the type abovediscussed, is eliminated.

In accordance with another feature of this inventi0n,the cathode andanode are constructed and arrangedso that during use of the devicesubstantially'all of the cathode emissivesurface is active-and thecurrent density thereat is low, whereby. a. low impedance isobtainedunder conditionsconducive to long cathode life.

The invention and the above-noted and other features thereof willbeunderstood more clearly and fully from the following detaileddescription with reference to the accompanying .drawing in which:

Fig. 1 is a view, partly diagrammatic, of a glow discharge device, whichwill be referred to hereinafter in an analysis of certain principlesinvolved in this invention;

Fig. 2 is a graph illustratingthe relation between gas pressure andcathode to anode spacing in glow discharge devices illustrative of thisinvention;

Fig. 3 is an elevational view of a glow discharge device constructed inaccordance with this in venticn, a portion of the enclosing vessel beingbroken away to show the internal structure more clearly;

Fig. 4 is a View of the electrodes in the device shown in Fig. 3 takenalong the plane fi-d of the latter figure;

Fig. 5 is a graph showing impedance characteristics of the deviceillustrated in Figs. 3 and l;

Fig. 6 is an elevational perspective view of another illustrativeembodiment of this invention,

a portion of the enclosing vessel being broken away;

Fig. 7 is an elevational view of still another embodiment of thisinvention, a portion of the enclosing vessel being broken away; and

Fig. 8 is a circuit diagram illustrating one manner in which devicesconstructed in accordance with this invention may be utilized.

Referring now to the drawing, the glow discharge device illustrated inFig.1 comprises an enclosing vessel It having therein an ionizableatmosphere, such as a rare gas, at a low pressure, for example of theorder of 50 millimeters of Mounted at one end of the vessel is an anodel l which may be a metallic disc. At the other end of the vessel is acathode [2, for example a disc like the anode, parallel thereto andhaving the face thereof toward the anode coated with a highly electronemissive material, such as a known mixture of barium and strontiumoxides.

A device of this type may be included in a speech or other intelligencetransmitting circuit as illustrated in Fig. 8. Specifically, the cathodeI2 and anode Il may be biased by direct current sources such asbatteries 13, relatively poled as shown, to produce between theelectrodes a potential sufficient to initiate and sustain a discharge.The device is connected between two stations, e. g. subscribersstations, by suitable transformers [4. The batteries are by-passed forthe alternating current signals to be transmitted, by condensers l5.

If a direct current potential ufficient to sustain a discharge isimpressed between the oathode and anode, it has been found that, ingeneral, the current through the device includes more or less randomvariations or noise components. These variations may be classified, onthe basis of oscillographic analyses, as (1) fluctuation noise, (2)spasmodic pulses of current and (3) an oscillatory type of noise.

The fluctuation noise, it has been ascertained, is due to the collisionof charged particles, i. e. electrons or ions, with other chargedparticles or with uncharged particles such as gas molecules, and to thearrival or emission of charged particles at the electrodes. Itsamplitude is rela tively small so that this type of noise is negligiblein many practical applications of glow discharge devices. For example,in a device of known construction it has been determined that this noiseis of the order of 45 decibels below the permissible noise level in alocal telephone transmission circuit.

The second type of noise, it has been ascertained, is associated withchanges in the position of various parts of the discharge, which will bedescribed in some detail hereinafter. Illustrative of such changes are ashift in the postition of the negative glow relative to the cathode whenall of the active cathode surface is not being utilized, and shifting ofthe positive column or anode glow. This type of noise is of substantialamplitude. It has been found that spasmodic current pulses due toshifting of the negative glow can be eliminated by operating the deviceunder such conditions that the direct current through the device issuflicient to bring all of the active cathode surface into use.

The third or oscillatory type of noise is of relatively large amplitude.It may be of complex wave form. However, it is definitely repetitive. Itappears impossible to eliminate such noise by changes in the externalcircuit associated with the device.

Noises of the second and third types, as has been indicated, are ofsubstantial level, and, hence, result in serious distortion of analternating current signals which are superimposed upon the directcurrent potential impressed across the device to initiate and sustain adischarge. For

xample, if a device of the type described is included in a telephonecircuit in such relation that speech or other audio frequency signalspass therethrough, the noise noted results in an intolerable signal tonoise ratio.

In accordance with one feature of this invention, noise of the thirdtype and also noise of the second type, particularly that associatedwith shifting of the positive column or anode glow, are eliminated.Specifically, this is effected by a unique correlation of the gas, gaspressure and cathode to anode spacing. It has been discovered that, fora given gas, for each gas pressure there is a critical maximum value ofcathode to anode spacing for which noise of the third type and of thesecond type associated with shifting of the anode glow or positivecolumn will not be produced and that conversely, for a given gas, foreach cathode to anode spacing there is a maximum gas pressure for whichsuch noise will not obtain.

The relationships for two typical gases in a device of the constructionshown in Fig. 1 are illustrated in Fig. 2, curve X being for a gasfilling of 100 per cent argon and the curve Y being for a gas filling ofper cent neon and 5 per cent argon. Each curve is based upon amultiplicity of noise measurements in devices such as shown in Fig. lbut wherein the anode was movable to vary the cathode to anode spacing.The coordinates of any point on each curve X and Y are the gas pressure(abscissa) and the corresponding maximum cathode to anode spacing(ordinate) for which noise of the third type will not obtain. As isapparent, the greater the gas pressure the closer must the anode bespaced relative to the cathode to result in elimination of the noise.Conversely, the smaller the anode to cathode spacing, the higher is thegas pressure which may be employed without the creation of noise. Thecorrelation of pressure and spacing in the manner illustrated in Fig. 2,it has been found, eliminates not only noise of the third type but alsonoise of the second type associated with shifting of the positive columnor the anode glow. The product of the pressure and the maximum spacingfor the prevention of noise is approximately constant for a given gas.

The relationship of gas pressure and cathode to anode spacing,illustrated in Fig. 2, requisite for absence of noise of the types aboveindicated has been found to obtain for a multiplicity of gases. Thefollowing table presents typical values illustrative of the relationshipfor a number of argon-neon mixtures in a device, such as shown in Fig.1, having parallel disc electrodes. The spacing given is the maximumcathode-anode spacing permissible at that given pressure for the absenceof oscillatory noise.

It has been established that, for a given gas and gas pressure, thecondition requisite for the elimination of the types of noise aboveindicated is that the anode be positioned in the Faraday dark space andso close to the cathode that anode glow does not obtain. That is to say,if in a glow discharge device, such as illustrated in Fig. 1, the anodeis moved toward the cathode, the anode position at which oscillatorynoise disappears is in the Faraday dark space and the beginning of theabsence of noise coincides with the disappearance of the anode glow. Theexplanation for this, it is believed, is found in the following analysisof the conditions extant in a glow discharge device.

Referringto'Fig. 1, if the anode is widely spaced from'the cathode andthe electrodes are energized to establish a discharge therebetween, thedischargepath comprises several distinct regions. Specifically,immediately adjacent the cathode is the cathode dark space C.successively this is followed, in the direction toward the anode, by thenegative glow region N, Faraday dark space F and positive column P.

In the cathode dark space, across which most of the voltage drop acrossthe device occurs, positive ions are produced and these bombard thecathode to cause release of electrons therefrom. Each electron thusreleased, under the influence of the potential gradient toward theanode, in its passage through the cathode dark space and into thenegative glow, causes a number of ionizations. At least some of the ionsthus produced flow back to and bombard the cathode, thereby to releaseother electrons. The process involved is repetitive sothat a dischargecan be sustained.

In the'negative glow region, there is considerable ionization, due toelectrons flowing from the cathode dark space. The ion and electronconcentrations are substantially equal so that the potential drop issmall.

By the time the electrons reach the Faraday dark space, their energy issubstantially spent. Consequently, there is but limited ionization inthis region. The ions and electrons in this space are supplied primarilyby diifusion from the negative glow. Their concentration decreases withincreasing distance from the negative glow. Also, both ions andelectrons pass to the wall of the enclosing vessel. Consequently, thefield in this region increases with increasing distance from the cathodeand a position is reached where the electrons have suiiicient energy toproduce sub stantial ionization. This position marks the beginning ofthe positive column.

In the positive column, there is approximately equal concentration ofelectrons and positive ions. A relatively small gradient in this regionis sufficient to cause ionization whereby ions and electrons areproduced to replenish those lost to the enclosing vessel.

If the anode is moved sufliciently close to the cathode, the positivecolumn may be caused to disappear, that is the anode is located in theFaraday dark space. The effects or conditions at the anode vary with theposition of the anode in this space. If the anode is located in theFaraday dark space and relatively remote from the negative glow, thenumber of positive ions thereadjacent is small so that an electron spacecharge and an anode drop build up. If this drop is sufficiently high,and it increases with distance from the cathode, excitation or anodeglow begins. This is followed by ionization. The ions produced reducethe electron space charge so that, as a, result, the potential drop issubstantially reduced or eliminated and the ionization ceases. Then theelectron space charge begins to build up and the cycle is repeated.Manifestly, the conditions are such as to produce oscillation wherebynoise of the third type heretofore noted is produced.

Now if the anode is moved still closer to the cathode, it will reach aposition, in the Faraday dark space, Where the ion and electronconcentrations are high. At this position, because of the ions, nosubstantial electron space charge obtains adjacent the anode when theelectrons are collected to produce the current in the external circuitbetween the anode and cathode. There 6 is, therefore, nosubstantialionization'or' excitation and no anode glow obtains. In the absence ofexcitation and ionization, oscillatory'noise -is not produced.

It is clear from the foregoing analysis,"and has been verified by testson actual devices, that there is a definite line of demarcation betweenanode positions, in-the- Faraday dark space, at which oscillation willor will not occur. As the anode is moved towardthe cathode, theoscillatory noise and anode glow disappear simultaneously. The conditionrequisite for prevention of oscillatory noise is 'that the'anode beinthe 'Faraday-dark space and so close'to the 'cathode that anode glowdoes not obtain.

it may be emphasized that thereis a-distinct difference between theanode-cathode spacings at which anode glow 'and the positive columnbegin. The relative locations of these two anode positions are'indicated at Po and -Pi=,-respectively, in Fig. 1.

The anode glow is affected by several factors, such as the electrodegeometry and-the anode area. In general, if the anode area is decreased,

the electron space charge density thereadjacent increases and the anodeglow and oscillatory noise appear at a smaller cathode-anode spacing.

Several electrode geometries for devices constructed in accordance withthe invention are illustrated in Figs. 3to 6. V

The glow discharge device illustratedin Figs. 3 and 4 comprises acylindrical cathodellZ, for example a nickel cylinder having a coatingof barium and strontium oxide mixture on its outer surface, and an anodecomprising apluralityof parallel, equally spaced wires or rods I l I 'ofiron arranged in a cylindrical boundary coaxial with the cathode. Thewires or'rods Ill aremounted and held in the prescribed space relationby a pair of metallic rings l6. Rigid conductors l7 embedded in the stem18 of the vessel Ill support the cathode and anode in coaxial relation.Significant parameters for a typical device of this constructionand freeof noise of the secondand third types heretofore described are:

Cathode diameter (outer)-% inch Cathode to anode spacing-.010 to.020'inch Gas99 per cent neon, 1 per cent argon Gas pressure'47 mm. ofmercury Sustaining voltage-53 volts A particular further feature of theglow discharge device illustrated in Figs. 3 and 4 is the low impedancethereof. It has been found, in general, that in cold 'cathodel'glowdischarge devices, for a steady direct current flowing through thedevice, the impedance of the device comprises both resistive andinductive components. It has been found further'that, ingeneral, theimpedance of the device increases as the current decreases.

Theoretically, the impedance of a glow discharge device may be decreasedby increasing the current flow through it. However, the life of commontypes of cold cathode devices with activated cathodesdecreases rapidlywith increase in such current. The impedance may be decreased somewhatalso by increasing the gas pressure. However, this not only results inan increase in the current density but also increases materially thedifliculty of obtaining a uniformly-active cathode surface.

The construction illustrated -in Figs. 3 and 4 enables attainment of lowimpedance witho'ut the deleterious effects and practical disadvantagesabove noted. Because of the large cathode surface and the juxtapositionof the anode elements thereto, the cathode may be operated at arelatively low current density, just large enough that all of thecathode emissive surface is used. The resistive and inductive componentsof the im pedance of a device of the construction shown in Figs. 3 and 4and having the parameters set forth above are illustrated, over a rangeof audio frequencies, by the curves R and Lw, respectively in Fig. 5.

In the device illustrated in Fig. 6, a rod anode 2 extends axially of acylindrical cathode M2, for example of sheet nickel having a coating ofa mixture of barium and strontium oxides on its inner surface and havingits outer surface calorized. In a specific embodiment, the cathode wasof it: inch inner diameter and the anode was of 0.030 inch diameternickel. For this construction, typical gas pressures which could beemployed without appearance of oscillatory noise were found to be asfollows:

In the embodiment of this invention illustrated in Fig. '7, the cathode312 and anode 3 are parallel discs rigidly supported from the stem [8 byleading-in conductors l9. Also supported from the stem l8 by leading-inconductors 20 encircled by insulating sleeves 2| are a pair of closelyadjacent auxiliary electrodes 22 and 23. These electrodes, which serverespectively as a control cathode and control anode, define a startergap by the controlled breakdown of which a discharge between the mainelectrodes 3]! and 3 l 2 may be initiated.

In a typical device of the construction illustrated in Fig. '7 and freeof oscillatory noise in the discharge gap between the main electrodes 3and M2, these electrodes were inch diameter discs, the cathode facetoward the anode was coated with a mixture of barium and strontiumoxides, these electrodes were spaced 0.25 inch and the gas Was argon ata pressure of millimeters of mercury. The main gap breakdown voltage was224 volts.

Although specific embodiments of the invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. A glow discharge device comprising an enclosing vessel having anionizable medium therein, and a cathode and an anode within said vessel,the cathode to anode spacing and pressure of said medium being suchthat, when a potential difference between said cathode and anodesulficient to sustain a discharge therebetween obtains, said anode is inthe Faraday dark space of the discharge and anode glow does not occur.

2. A glow discharge device in accordance with claim 1 wherein said anodeand cathode have plane, parallel opposed faces.

3. A glow discharge device in accordance with claim 1 wherein said anodeand cathode are cylindrical.

4. A glow discharge device comprising an enclosing vessel having agaseous filling, and a cathode and an anode in parallel relation withinsaid vessel, said anode being spaced such distance from said cathodethat when a potential difierence between said cathode and anodesufficient to sustain a discharge therebetween obtains, the anode is inthe Faraday dark space of the discharge and anode glow does not occur.

5. A glow discharge device comprising a cathode and an anode, and anenvelope enclosing said cathode and anode and having a fillingcomprising a rare gas therein, said anode being spaced from the cathodea distance such that when a sustained discharge exists between thecathode and anode the anode is in the Faraday dark space of thedischarge, and said distance being less than that for which substantialionization immediately adjacent said anode occurs.

6. A glow discharge device in accordance with claim 5 wherein saidcathode and anode are plane and parallel.

7. A glow discharge device in accordance with claim 5 wherein saidcathode and anode are cylindrical.

8. A glow discharge device in accordance with claim 5 wherein said raregas is argon.

9. A glow discharge device comprising an enclosing vessel having thereina filling of argon at a pressure in the range between about 5 and 35millimeters of mercury, and a cathode and an anode Within said vesseland spaced a distance less than about 0.64 inch.

10. A glow discharge device comprising an enclosing vessel havingtherein a mixture of substantially per cent neon and 5 per cent argon ata pressure in the range between about 20 and 60 millimeters of mercury,and a cathode and an anode within said vessel and spaced a distance lessthan about 0.46 inch.

11. A glow discharge device comprising an enclosing vessel having agaseous filling, a cylindrical cathode within said vessel, and an anodecomprising a plurality of parallel linear elements mounted in acylindrical boundary coaxial with said cathode, the pressure of saidfilling and the spacing of said elements relative to said cathode beingsuch that when a potential sufficient to sustain a discharge betweensaid cathode and anode obtains, the anode is positioned in the Faradaydark space of the discharge and anode glow does not occur.

12. A glow discharge device in accordance with claim 11 wherein saidgaseous filling is about 99 per cent neon and 1 per cent argon at apressure of the order of 47 millimeters of mercury and said cathode andanode are spaced approximately 0.015 inch.

13. A glow discharge device comprising an enclosing vessel having agaseous filling, a cylindrical cathode within said vessel, and a rodanode within and extending along the axis or said cathode, the pressureof said filling and the distance between said anode and cathode beingsuch that when a potential between said cathode and anode sufficient tosustain a discharge therebetween obtains, the anode is located in theFaraday dark space of the discharge and anode glow does not occur.

14. A glow discharge in accordance with claim 13 wherein said filling isargon at a pressure of substantially 10 to 24 millimeters of mercury,the

inner diameter of said cathode is substantially inch and the diameter ofsaid anode is substantially 0.03 inch.

15. A glow discharge device comprising an enclosing vessel having agaseous filling, a plane cathode and a plane anode in parallel relationwithin said vessel and defining a main discharge gap, and auxiliaryelectrode means within said vessel forhcontrolling the initiation of adischarge across said main gap, the pressure of said filling and thedistance between said cathode and said anode being such that when apotential is impressed between said cathode and anode to sustain adischarge therebetween, the anode is within the Faraday dark space ofthe discharge and so close to the cathode that no substantial ionizationobtains immediately adjacent said anode.

16. A glow discharge device in accordance with claim 15 wherein saidgaseous filling is argon at a pressure above substantially 5 millimetersof mercury and less than about 35 millimeters and said distance is lessthan 0.64 inch.

17. The method of constructing a glow discharge device including acathode and an anode in a gas-filled enclosing vessel, which comprisesimpressing between said cathode and anode a potential sufficient toestablish a discharge therebetween, and adjusting the distance betweensaid cathode and anode until the anode is in the Faraday dark space ofthe discharge and at such REFERENCES CITED 'The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,603,420 Schroter Oct. 19, 19261,735,080 Hertz Nov. 12, 1929 1,900,577 Moore Mar. 7, 1933 1,965,582Foulke July 10, 1934 1,995,018 Spanner Mar. 19, 1935 2,331,398 IngramOct. 12, 1943 FOREIGN PATENTS Number Country Date 209,969 Germany Nov.10, 1908 349,921 Germany Mar. 10, 1922

